High pressure sodium arc discharge lamps have been in commercial production for many years and have been subject to many improvements in design, materials and processing. Such lamps include a translucent ceramic arc tube, a light-transmissive lamp envelope, a base connector and a frame for supporting the arc tube within the lamp envelope. The frame is electrically conductive and carries power to the arc tube. The arc tube is typically fabricated of polycrystalline alumina or yttria and contains an amalgam of mercury and sodium for producing light having a desired output spectrum. Tungsten or molybdenum electrodes are mounted within the arc tube at opposite ends and are attached to feedthroughs selected to have thermal expansion characteristics closely matched to those of the ceramic arc tube. The feedthroughs are hermetically sealed in openings at opposite ends of the arc tube. Niobium, usually containing about 1% zirconium by weight, is the preferred feedthrough material for alumina arc tubes.
In one commonly used electrode feedthrough structure, the feedthrough is a niobium tube. A tungsten coil electrode is attached to the niobium tube by a tungsten support rod. The opening in each end of the arc tube is sufficiently large for insertion of the electrode and the niobium tube. An insert button is sintered directly into the end of the arc tube, and a ceramic sealing button or ring is sealed with a low melting point ceramic frit to the end of the arc tube and the feedthrough to extend the length of the seal and to improve its reliability.
In one prior art method for manufacturing arc tubes, an electrode assembly is sealed in one end of the arc tube, and the required chemical fill is introduced into the arc tube through the open end. Then, the end cap containing an electrode assembly is bonded to the other end of the arc tube. The niobium tube of the second electrode assembly is used as an exhaust tube. The exhaust tube is connected to a system used to purge the arc tube of undesirable gaseous components and to permit the addition of the required gas fill. After the gas filling operation, the exhaust tube is sealed, typically by crimping and welding.
Another prior art technique for manufacturing arc tubes involves sealing an electrode assembly and end cap into one end of the arc tube. The chemical fill for the arc tube is introduced through the open end of the arc tube. The second end cap and the electrode assembly are then loosely positioned on the open end of the arc tube, and the arc tube is placed in a chamber that can be purged of undesired gases and then refilled with the desired gas. Then the region of the second end cap is heated, causing the cap to be hermetically sealed to the arc tube body. An advantage of this process is that it permits batch processing of many units at one time.
Smaller and lower wattage high pressure sodium arc tubes have recently been developed for various applications. The above-described arc tube manufacturing processes have been found unsuitable for manufacturing such smaller and lower wattage arc tubes. One problem is that an excessive amount of heat is transferred during sealing of the second end cap and electrode assembly to the volatile chemical fill material. This results in vaporization of the chemical fill and migration of the chemical fill out of the arc tube before it is sealed. It has been found that the smaller the length of the arc tube, the greater the tendency to lose its chemical fill during processing. Proposed methods to shorten the thermal cycle or to reduce heat transfer to the chemical fill have been only partially successful.
It is known in the prior art to construct high pressure sodium arc tubes so that the interior of the electrode feedthrough tube is connected to the discharge region in the ceramic arc tube by a passage of sufficient cross section to permit flow of the vaporized fill material. The interior of the feedthrough tube is usually lower in temperature than the discharge region of the arc tube. Therefore, the fill material tends to condense in the feedthrough tube. This construction is commonly referred to as an external reservoir arc tube, since the fill material condenses in a region external to the discharge region.
External reservoir construction is disclosed in U.S. Pat. No. 4,342,938 issued Aug. 3, 1982 to Strok, European Patent application No. 0,225,944 published Jun. 24, 1987, U.S. Pat. No. 4,827,910 issued May 2, 1989 to Masui et al, European Patent application No. 0,265,266 published Apr. 27, 1988, U.S. Pat. No. 4,035,682 issued Jul. 12, 1977 to Bubar and U.S. Pat. No. 4,065,691 issued Dec. 27, 1977 to McVey. The external reservoir arc lamp construction is believed to provide lower sodium loss than conventional arc lamps and to provide a more constant level of light output over the life of the arc lamp.
It is a general object of the present invention to provide improved high pressure arc discharge lamps.
It is another object of the present invention to provide improved methods for manufacturing high pressure arc discharge lamps.
It is a further object of the present invention to provide methods for transferring a chemical fill into an electrode feedthrough tube of an arc discharge lamp.
It is a further object of the present invention to provide methods for manufacturing arc discharge lamps wherein loss of chemical fill during processing is substantially reduced.